1. Field of the Invention
The present invention relates to acoustic wave devices, resonators and filters, and more particularly, to an acoustic wave filter having a dielectric film that covers comb electrodes, and a resonator and a filter using the acoustic wave filter.
2. Description of the Related Art
A filer using an acoustic wave is used in high-frequency radio equipment such as a cellular phone. An exemplary acoustic wave is a surface acoustic wave (SAW). FIG. 1 is a plan view of a SAW resonator, and FIG. 1B is a cross-sectional view taken along a line A-A′ shown in FIG. 1. FIG. 1A shows a smaller number of fingers of an electrode 12 than an actual number for the sake of simplicity. The electrode 12 is formed on a piezoelectric substrate 10, which may be a lithium tantalate (LiTaO3) substrate or a lithium niobate (LiNbO3) substrate. The electrode 12 may be made of an aluminum alloy, a copper alloy or gold. The electrode 12 includes a pair of comb electrodes IDT0 and a pair of reflection electrodes RO. The pair of comb electrodes IDT0 forms an interdigital transducer. The reflection electrodes R0 include grating electrodes. The pair IDT0 of comb electrodes is provided between the reflection electrodes R0. When a high-frequency signal is applied to one of the pair IDT0 of comb electrodes, a surface acoustic wave is excited on the surface of the piezoelectric substrate 10. The SAW is resonated at a frequency that depends on the period λ of the electrode fingers of the pair IDT0 of comb electrodes and the propagation velocity of the SAW. A high-frequency signal of the resonance frequency is developed at the other comb electrode of IDT0. Thus, the device shown in FIGS. 1A and 1B functions as a resonator.
In practice, the SAW device is required to have a reduced absolute value of a temperature coefficient of frequency (frequently abbreviated as TCF). TCF is a rate of change of the frequency response to a variation in the environment temperature. In the resonators, a change of the resonance frequency to a variation in the environment temperature equal to 1° C. is expressed in the unit of ppm/° C. TCF almost depends on the temperature coefficient of velocity of SAW propagated on the surface of the piezoelectric substrate. The TCF of the SAW device is as bad as −80˜−40 ppm/° C. for a piezoelectric substrate of LiNbO3 or LiTaO3, and is thus required to be improved.
The following documents disclose techniques directed to improving the TCF of SAW devices: International Publication No. WO98/52279 (hereinafter referred to as D1), and Masatsune Yamaguchi, Takashi Yamashita, Ken-ya Hashimoto, Tatsuya Omori, “Highly Piezoelectric Boundary Waves in Si/SiO2/LiNbO3 structure”, Proceeding of 1998 IEEE International Frequency Control Symposium, IEEE, 1998, pp. 484-488. Referring to FIG. 2, the electrode 12 is covered with a first dielectric film 14, which may be made of silicon oxide and may be as thick as 0.2λ to 0.4λ. An acoustic wave called Love wave is excited in the acoustic wave device shown in FIG. 2. The Love wave has changes in both the piezoelectric substrate 10 and the first dielectric film 14. Referring to FIG. 3, a third dielectric film 16, which may be made of aluminum oxide, is provided on the first dielectric film 14. The acoustic wave device shown in FIG. 3 has an excited acoustic wave called boundary wave having changes in both the piezoelectric substrate 10 and the first dielectric film 14. In the SAW devices shown in FIGS. 2 and 3, the acoustic wave is propagated in not only the piezoelectric substrate 10 but also the first dielectric film 14. The temperature coefficient of propagation velocity of the acoustic wave propagated in the first dielectric film 14 is designed to have a sign opposite to that of the temperature coefficient of propagation velocity of the acoustic wave propagated in the piezoelectric substrate 10. When the thickness of the first dielectric film 14 is optimally selected, the total propagation velocity of acoustic wave can be kept almost constant. That is, TCF can be reduced by optimally selecting the thickness of the first dielectric film 14.
However, the acoustic wave devices shown in FIGS. 2 and 3 has a problem that the mechanical resonance sharpness Qm of the first dielectric film 14 is considerably smaller than that of the piezoelectric substrate 10. Thus, loss is caused in the acoustic wave propagated in the first dielectric film 14, so that the acoustic wave device has a large loss.